This invention relates to the technical field of optical elements such as lens and prism and, more particularly, to plastic optical elements that undergo extremely small changes in characteristics upon absorbing moisture (water).
Conventionally, optical elements such as camera lens or viewfinders, various other kinds of lenses and prisms that are employed in copying equipments, printers, projectors and optical communications, as well as optical elements including eyeglass lens, contact lens and magnifier are in most cases manufactured from glass. However, with recent advances in the plastic forming technology, it has become popular to manufacture lenses and prisms from plastics that can be mass-produced from inexpensive starting materials and which are lightweight.
However, plastics have the disadvantage that their refractive indices and shapes will change upon moisture absorption. Because of these constraints, plastics are not suitable for use as the material of optical elements that require high precision, an example of which is a lens in a high-quality single-lens reflex camera.
With a view to solving this problem, attempts have been made to develop plastic materials having high moisture-proof quality (absorbing less moisture) through polymer structure design and other strategies; however, the kinds of promising plastic materials are so few that the degree of freedom in the design of optical elements is quite limited in various terms including cost. It is known to coat a surface of an anti-reflective layer of optical parts with a different material than the anti-reflective layer to a thickness of 0.5-20 nm and apply a water- and oil-repelling treatment on the anti-reflective layer (JP 2002-148402 A).
However, what is applied in this treatment is a single-layered coat of a silane coupling agent and it has no ability to prevent moisture absorption although it can repel water. In order to make plastic optical elements having moisture-proof quality, it is often attempted to add hydrophobic substances into the optical element as it is being molded or else, or cover the molded optical element with a barrier film impermeable to moisture. However, these methods are still incapable of producing plastic optical elements having adequate moisture-proof quality.